Development of a solid oxide fuel cell, having a laminate structure in which a solid electrolyte layer made of an oxide ion conductor is sandwiched between an air electrode layer (oxidant electrode layer) and a fuel electrode layer, is progressing as third-generation fuel cell for use in electric power generation. In a solid oxide fuel cell, oxygen (air) is supplied to an air electrode section and a fuel gas (H2, CO and the like) is supplied to a fuel electrode section. An air electrode and a fuel electrode are both made to be porous so that gases can reach interfaces in contact with the solid electrolyte layer.
Oxygen supplied to an air electrode section passes through pores in the air electrode layer and reaches a neighborhood of the interface in contact with the solid electrolyte layer, and in that portion, the oxygen receives electrons from the air electrode to be ionized into oxide ions (O2−) . These generated oxide ions move in the solid electrolyte layer by diffusion toward the fuel electrode. The oxide ions having reached the neighborhood of the interface in contact with the fuel electrode react with fuel gas in that portion to produce reaction products (H2O, CO2 and the like), and release electrons to the fuel electrode.
The electrode reaction when hydrogen is used as fuel is as follows:
Air electrode: ½ O2+2e−→O2−
Fuel electrode: H2+O2−→H2O+2e−
Overall: H2+½ O2→H2O
Because the solid electrolyte layer is a medium for migration of the oxide ions and also functions as a partition wall for preventing direct contact of the fuel gas with air, the solid electrolyte layer has a dense structure capable of blocking gas permeation. It is required that the solid electrolyte layer has high oxide ion conductivity, and is chemically stable and strong against thermal shock under conditions involving an oxidative atmosphere in the air electrode section and a reductive atmosphere in a fuel electrode section. As a material which can meet such requirements, generally a stabilized zirconia (YSZ) that is added with yttria is used.
On the other hand, the air electrode (cathode) layer and fuel electrode (anode) layer need to be formed of materials having high electronic conductivity. Because an air electrode material is required to be chemically stable in an oxidative atmosphere of high temperatures around 700° C., metals are unsuitable for the air electrode, and generally used are perovskite type oxide materials having electronic conductivity, specifically LaMnO3 or LaCoO3, or solid solutions in which part of an La component in these materials is replaced with Sr, Ca and the like. Moreover, the fuel electrode material is generally a metal such as Ni or Co, or a cermet such as Ni—YSZ or Co—YSZ.
A solid oxide fuel cell is classified into a high temperature operation type operated at high temperatures around 1000° C., and a low temperature operation type operated at low temperatures around 700° C. A solid oxide fuel cell of the low temperature operation type uses an electric power generation cell which is improved to work as a fuel cell even at low temperatures by lowering a resistance of an electrolyte, for example, through making the electrolyte made of an yttria stabilized zirconia (YSZ), be a thin film on the order of 10 μm in thickness.
A solid oxide fuel cell operable at high temperature is used for the separator, for example, a ceramic having electronic conductivity such as lanthanum chromite (LaCrO3), while a solid oxide fuel cell of a low temperature operation type can be used for the separator, i.e. a metallic material such as stainless steel.
Additionally, as structure of the solid oxide fuel cell, there have been proposed three types, namely, a cylindrical type, a monolithic type and a flat plate type.
A stack of a solid oxide fuel cell has a structure in which electric power generation cells, current collectors and separators are alternately laminated. A pair of separators sandwich an electric power generation cell from both sides of the cell in such a way that one of the separators is in contact with the air electrode through intermediary of an air electrode current collector, while the other separator is in contact with the fuel electrode through intermediary of a fuel electrode current collector. For the fuel electrode current collector, a spongy porous substance made of a Ni based alloy or the like can be used, while also for the air electrode current collector, a spongy porous substance made of a Ag based alloy or the like can be used. A spongy porous substance simultaneously displays a current collection function, gas permeation function, uniform gas diffusion function, cushion function, thermal expansion difference absorption function and the like, and is accordingly suitable for a multifunction current collector.
The separators electrically connect between electric power generation cells, and also have a function to supply gas to the electric power generation cells. Therefore, each separator has a fuel path through which fuel gas is introduced from a peripheral side of the separator and is discharged from a separator surface facing the fuel electrode layer, and an oxidant path through which oxidant gas is introduced from the peripheral side of the separator and is discharged from a separator surface facing the oxidant electrode layer.
<Problems to be Solved by the Invention>
<First Problem>
In a case of the solid oxide fuel cell of the low temperature operation type, metal (stainless steel or the like) plates on the order of 5 to 10 mm in thickness are used for the separators, and there has hitherto been known a separator having a structure such that gas discharge openings, to discharge fuel gas and oxidant gas introduced from the peripheral side of the separator into the current collector, are provided in a central part of the separator.
FIG. 8 is a sectional view of a relevant portion of a fuel cell stack illustrating an example of the above described separator. In FIG. 8, reference numeral 3 denotes a fuel electrode layer, reference numeral 6 denotes a fuel electrode current collector, reference numeral 8 denotes a separator, reference numeral 11 denotes a fuel path, reference numeral 25 denotes a gas discharge opening, and arrows indicate a gas permeation condition.
Here, it should be noted that such a conventional separator structure as described above is associated with the following problems.
More specifically, the structure is such that fuel gas discharged from the central part of the separator 8 is supplied to an entire area of the fuel electrode layer 3 through the fuel electrode current collector 6 made of a porous cushioning material; however, in practice, there is a problem in that the fuel gas is consumed to a large extent by an electrode reaction in a neighborhood of the gas discharge opening 25, and hence a gas concentration is decreased with increasing distance away from the gas discharge opening 25. Consequently, the electrode reaction is not uniformly conducted over the entire area of the electrode, a temperature gradient is thereby generated in the electric power generation cell, the electric power generation cell is sometimes broken down by thermal stress thus generated, and a resulting inefficient electric power generation leads to degradation of electric power generation properties (electricity production comes to be large in a central part of the electric power generation cell and small in a peripheral part of the same cell) . This problem has been particularly conspicuous in the fuel electrode section.
Additionally, use of thick metallic plates of 5 to 10 mm in thickness makes a weight of a single cell itself great, and accordingly, in a case of a solid oxide fuel cell constructed by longitudinally arranging cell stacks, there is a problem such that the electric power generation cells in the cell stacks located in a bottom portion tend to be broken by a weight of the fuel cell. Consequently, as affairs stand, there remains a problem in that a cell configuration is inevitably constrained in such a way that a number of laminations is consistent with a tolerable weight of the fuel cell. Incidentally, in a case of a conventional structure, a weight of a cell stack is about 1 kg, and a total weight of a cell module made by laminating a large number of these cell stacks comes to be about 25 kg. Consequently, a structure supporting such a module is naturally complex.
<Second Problem>
As described above, in a conventional solid oxide fuel cell, each of the current collectors made of a porous cushioning material is arranged between an electrode layer and a separator, and gas is distributed to be supplied to each of the electrode layers through the current collectors; however, there has been a problem in that in the conventional structure, a retaining time of the gas in a current collector is short, and consequently fuel gas not engaging with the electrode reaction is discharged outside the electric power generation cell, so that electric power generation efficiency is thereby degraded.
Additionally, in the conventional structure, a linear velocity of gas in the peripheral part of the electric power generation cell comes to be low. Consequently, there has also been a problem in that from the peripheral part of the electric power generation cell, air as oxidant is taken into an interior of the electric power generation cell, where a combustion reaction tends to take place, the combustion reaction completely consumes the fuel gas to be usable for the electrode reaction, and consequently electric power generation efficiency is degraded.
Such an adverse phenomenon has remarkably taken place particularly in a fuel cell stack provided with separators having a structure in which fuel gas or oxidant gas is supplied to the fuel cell electrode current collector or the oxidant electrode current collector from a central part of each separator.